A need presently exists for improved tackifying resins for use in thermoplastic adhesives formed of metallocene catalyzed polyolefins and other aliphatic base polymers.
Until somewhat recently, hot melt adhesives (HMAs) formed of ethylene vinyl acetate (EVA) were predominantly used in the packaging industry and in other applications. However, the introduction of metallocene catalyzed polyolefin elastomers in the late 1990s led to the development of new HMAs which are superior to the EVA-based adhesives in both their machineability and in their relative oxidation resistance and clarity.
Adhesives formed of metallocene catalyzed polyolefin elastomers and other aliphatic base polymers are used, for example, for packaging (e.g., case or carton sealing, flexible packaging, etc.), product assembly applications, and pressure sensitive applications. The metallocene polymers can provide improved thermal stability at high and low temperatures, improved processability, machineability, reduced crystallinity, lower color, and lower molecular weight.
As with other hot melt and pressure sensitive polymer adhesives compositions, the aliphatic polymer based adhesive formulations also include tackifying resins which increase the adhesion (i.e., the ability of the adhesive to form a lasting bond) of the adhesive to the substrate. These tackifiers tend to have low molecular weights (relative to the base polymer), as well as glass transition and softening temperatures above room temperature, which give the tackifiers unique amorphous properties. Tackifiers frequently account for a significant portion of the weight of a hot melt adhesive product as they can comprise up to 50% or more of the total mass of the product.
Heretofore, rosin ester tackifiers or oligomers have been successfully used to improve the adhesion properties of various polymer adhesive compositions. Unfortunately, however, these rosin ester tackifiers have not been compatible with metallocene catalyzed polyolefins and other aliphatic base polymers. When blended with these polymers, the blended system turns opaque white and forms separate microphases which separate very quickly. Fundamentally, the use of rosin ester tackifiers in metallocene polymer compositions has been limited due to the composition and structure of the ester as well as the disproportionation reactions which occur when the rosin is heated to a temperature above 230° C. During the disproportionation step of the rosin, either thermally or in the presence of a disproportionation catalyst, the isomers of rosin convert from the typical abeitic, palustric, and neoabietic types to more stable dehydroabietic isomers. These isomers, with their terminal fused aromatic ring structure increase the polarity of the resin which furthers the rosin esters incompatibility with the aliphatic base polymer.
Consequently, heretofore, the tackifying resin materials used in metallocene catalyzed polyolefin and other aliphatic base polymer compositions have been hydrogenated hydrocarbon tackifiers or oligomers. In comparison to rosin ester tackifiers, these hydrogenated hydrocarbon resins, when formulated into typical adhesives, have higher melt viscosities, and reduced formulation latitude. Moreover, and more importantly, the hydrogenated hydrocarbon resins have less robust adhesion characteristics as compared to rosin ester tackifiers when evaluated in similar formulations.
Therefore, in view of the above, it will be apparent that a need exists for: (a) an improved tackifying resin for metallocene catalyzed polyolefins and other aliphatic base polymers; (b) a rosin ester tackifier composition which is compatible with metallocene catalyzed polyolefins and other aliphatic base polymers; and (c) an improved metallocene catalyzed polyolefin adhesive composition comprising the new rosin ester tackifier composition.